91av

WRAPPING submarines in a cloak of tiny waves created by a smart “skin” could
help them slip swiftly and silently through the water, say researchers in
Massachusetts and Rhode Island.

Fluid flowing past a solid object, such as the hull of a submarine, tends to
get twisted into turbulent eddy currents. This turbulence increases drag, so
it’s harder for the submarine to move through the water. Naval architects wage a
constant war against turbulence, designing hulls that are as streamlined as
possible. But Yiquing Du of the Massachusetts Institute of Technology in
Cambridge and George Karniadakis of Brown University in Providence, Rhode
Island, have hit on a way to stop turbulence developing in the first place.

Their approach is to create tiny waves that travel across the surface of the
solid object at right angles to the fluid flowing past it. These waves
effectively wrap the object in a watery “envelope”, so that the water flowing
past hits this soft, wet buffer, rather than a hard surface. “It shields the
surface from the outer flow,” explains Karniadakis. “You cut off the main
mechanism of producing turbulence.”

According to their computer simulation you only need waves a few micrometres
high to achieve a striking effect. “We have at least 30 per cent drag reduction
and we think we can double this,” says Karniadakis. This beats the best existing
method of eliminating turbulence, which uses tiny ribs to alter flow near the
surface and only reduces drag by 10 per cent
(91av, 18 January 1997, p 28).

The team have also tried out their idea in a test tank where electromagnets
push and pull a mosaic of tiles, to make waves travel across their surface.
Ultimately they want to create a “smart skin” of tiny wave-making machines on
the hull of a submarine, for example. Piezoceramic materials, which flutter very
rapidly in response to a voltage, would be ideal for the job, according to
Karniadakis.

This smart skin could be applied to existing hulls without any remodelling.
“These are very thin materials,” notes Karniadakis, “This doesn’t require
complicated contouring of surfaces.” And since the waves are so tiny, they
shouldn’t require much energy to make them. “You don’t need to move a very large
volume of water,” he says. Karniadakis and his colleague Kenneth Breuer at Brown
University are developing the technology with research funding from the US
Department of Defense.

“The plan is to have a prototype submerged vehicle and see how fast it goes,”
says Karniadakis. As well as increasing speed, eliminating turbulence could let
a submarine move through the water more quietly and leave less telltale
disturbance behind. “The Navy are very interested in the noise aspect,” says
Karniadakis.

So far they’ve been able to reduce drag in simulations only at comparatively
low flow speeds, as modelling high speeds needs massive number crunching. “But
there isn’t any `fudge’ in the calculation,” comments Kwing-So Choi of the
University of Nottingham. However, Choi—who works on reducing drag in
pipelines using a different kind of wave—would like to see experiments
showing the effect at high flow speeds.